LTR causes #GP

[antoni]

I'm currently working on dispatcher. I've already implemented few things like switching stack, address space, segment registers, restoring registers etc. Now I'm trying to implement user mode. I'm using our wiki's tutorial for this (https://wiki.osdev.org/Getting_to_Ring_3), except that I'm doing this in long mode so I have 64bit TSS, different iret stack frame etc. (I want to use the iret method).

I really don't know why LTR gives me #GP.

Here's the code:

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mov ax, (5 * 8) | 3
ltr ax

Saved rip (in #GP handler) points to "ltr ax" instruction.

Here's my TSS:

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tss = {
    limit_low = 0x1000, base_low = 0x5000, base_middle = 0x11, 
    access = 0xe9, granularity = 0x0, base_high = 0x0}

access = 0xE9 = 11101001 =
accessed: 1
read_write: 0
conforming_expand_down: 0
code: 1
code_data_segment: 0
DPL: 3 = 11
present: 1

Here you have my whole GDT:

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{null = {limit_low = 0xffff, base_low = 0x0, base_middle = 0x0, 
    access = 0x0, granularity = 0x1, base_high = 0x0}, code0 = {
    limit_low = 0x0, base_low = 0x0, base_middle = 0x0, access = 0x9a, 
    granularity = 0xaf, base_high = 0x0}, data0 = {limit_low = 0x0, 
    base_low = 0x0, base_middle = 0x0, access = 0x93, granularity = 0x8f, 
    base_high = 0x0}, code3 = {limit_low = 0x0, base_low = 0x0, 
    base_middle = 0x0, access = 0xfa, granularity = 0xaf, base_high = 0x0}, 
  data3 = {limit_low = 0x0, base_low = 0x0, base_middle = 0x0, 
    access = 0xf2, granularity = 0x8f, base_high = 0x0}, tss = {
    limit_low = 0x1000, base_low = 0x5000, base_middle = 0x11, 
    access = 0xe9, granularity = 0x0, base_high = 0x0}, pointer = {
    limit = 0x2f, base = 0x100100130}}

[nullplan]

In 64-bit long mode, the TSS descriptor is a long descriptor (spans two entries), where the upper entry contains only the high 32 bits of the TSS base address. This is one reason why I just declare the GDT as array of 64-bit integers. Here's my code:

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enum {
    NULL_DESC,
    KCODE_DESC,
    KDATA_DESC,
    UCODE_DESC,
    UDATA_DESC,
    TSS_DESC,   /* TSS is long */
    TSSU_DESC,
    MAX_GDT
};
static void init_gdt(uint64_t *gdt, const struct tss* tss)
{
    uint64_t tssp = (uint64_t)tss;
    gdt[KCODE_DESC] = 0x00af9a000000ffff;
    gdt[KDATA_DESC] = 0x00af92000000ffff;
    gdt[UCODE_DESC] = 0x00affa000000ffff;
    gdt[UDATA_DESC] = 0x00aff2000000ffff;
    gdt[TSS_DESC] = (tssp << 16) & 0xffffff0000 | (tssp << 32) & 0xff00000000000000 | sizeof (struct tss) - 1 | (uint64_t)0x89 << 40;
    gdt[TSSU_DESC] = tssp >> 32;
}

The same is true for other system descriptors, like LDTs, but I don't use any of those.

[antoni]

Wow! It worked. Thanks a lot!